In many industrial environments, bulk solid products are stored and/or processed in silos, tanks or bins. Examples include grain and compound feed silos, and tanks used in the batch processing of foods, pharmaceuticals, sand or minerals. In these industries, one must be able to reliably determine the amount of content stored within a container at any given moment to provide an adequate supply and monitor the in/out flow of content. This determination may be made visually. However, in many instances, the container prevents any sort of visual determination of the present level of the contents. For example, many silos are composed of steel and/or concrete or other nontransparent materials and, therefore, any visual inspection of the content level of such containers would involve manually opening an aperture, which may pose hazards to personnel, and raise the potential of contamination of the contents. Furthermore, visual inspection of content levels lacks accuracy, is time consuming and prevents the automatization of processes. In order to avoid these problems, there are different types of devices and methods that are commonly used to measure the amount of content present in a container.
One method is to determine the mass of the content using load cells or strain gauges installed in the container's support structure, subtracting to the measured mass the known mass of the empty container. Although this method is quite accurate it is not suitable in many occasions due to the high cost of the sensors used, the requirement of a support structure in the silo, the difficulty of the installation and the need of emptying the container to calibrate the system.
An alternative to the previous method is to determine the level of the content using level sensing gauges installed on a top part of the container, subtracting to the known container's height the measured distance between the sensor and one point of the surface of the content. The level measurement can be done by contact means (cable, guided wave radar, etc.) or by non-contact means (ultrasound, radar, laser, etc.). The advantage of these systems is their lower cost, compared with load cells, and that they are easier to install and calibrate. The main problem is a lack of accuracy when they are used in containers with bulk solids, with measurement errors equivalent up to 15-20% of the total container's capacity, due to the uneven surface of this types of contents.
To mitigate the lack of accuracy of level sensors when used in containers with bulk solids, there are several known methods. One is to install several level sensors in the same container and calculate the level as the mean of the levels measured in different points. It has been also disclosed the possibility of using a single non-contact level sensor capable of measuring several points, by means of an actionable gimbal mechanism or any other alternative, and proceed as in the previous case calculating the level as the mean of the levels measured in different points. Although these methods provide better accuracy, they are far from the accuracy provided by load cells and are unable to measure the exact amount of the content stored in the container or detect small variations in the distribution of the content. Furthermore, the installation and calibration process of these systems are quite more difficult than single-level systems, because the multi-point sensor or set of sensors must be precisely oriented in a way that the obtained measures represent a good sample of the content throughout all the level range. The system must also be calibrated in some way that can be discarded the points that correspond to the container interior walls, which is an important issue when the observed area is broad and in irregular containers.
It is also known the method of calculating the volume of the container's content using the given type and dimensions of the container and the measure of the level obtained by the sensor. Additionally, it can be obtained the mass of the content using the calculated volume and a given density of the content. Although these methods work, they are dependent of the accuracy of the measured level, of the given parameters of the container's dimensions and of the homogeneity of the density.
Some patents or patents applications are already known in the field for measuring the amount of content in containers.
U.S. Pat. No. 7,515,095 B2 discloses an antenna for a level sensing gauge that comprises an environmentally sealed chamber extending from the level sensing gauge and mounting and positionable within the interior of a storage container, including a breathable aperture for permitting pressure and humidity equalization between the interior of said chamber and an exterior environment. According to this patent, the sensing gauge is a radar sensing gauge which is mounted to the exterior wall of the storage container by a mounting and antenna system based on microwave technology.
US-A1-2007040677 relates to a cargo detection unit that detects the status of cargo and loading activity within a container. The device transmits microwave radio frequency energy pulses and detects reflections from cargo. Similar to the operation of a radar, the reflected pulses are then analyzed to determine (a) the presence of cargo, such as by comparing the reflected pulses against stored empty containers signature signals and/or (b) detecting a Doppler effect, as caused by loading and/or unloading cargo from the container. The device may use standard radar signal processing techniques, i.e., a digital signal processor, to generate and analyze the reflected pulses cargo status. Activity reports can be forwarded to a cargo tracking unit such as one that uses a wireless mobile telephone communication network to report cargo status to a central location.
U.S. Pat. No. 6,986,294 B2 discloses bulk material measurement packages including an automated instrument package (AIP) system comprised of different sensors suited to be mounted on the inside ceiling of a large silo. In this patent, a laser rangefinder (TOF or phased-based) which sends out pulses of infrared or visible light to obtain reflections off a desired surface may be used. The AIP system of sensors is designed for the on-site user as well as the needs of other parties remotely located from the storage site
US-A1-2005080567 discloses a grain bin monitoring system for efficiently monitoring remote grain bins. The grain bin monitoring system includes a main station, a central unit in communication with the main station, a plurality of transmitter units in communication with the central unit, and at least one sensor positionable within a grain bin for determining condition data with respect to a grain bin. The sensor is in communication with one of the transmitter units for providing the condition data to the transmitter unit, wherein the transmitter unit automatically forwards the condition data to a central unit that automatically forwards the condition data to the main station. In the event of an alarm condition, an individual may be notified.
U.S. Pat. No. 8,820,182 B2 relates to the remote monitoring of the floating roofs of large storage tanks, including tanks used for storing liquid petroleum products or other chemicals. The invention comprises one or more intelligent sensor units and one or more communication units. The sensor unit integrates multiple sensors into a self-contained unit that can be completely encapsulated for use in harsh environments. Additionally, the unit may have a magnetic base for rapid installation on steel tank roofs. The communication unit communicates with the sensor units and with an external monitoring system. The communication unit can be used to relay information from the sensor units to or from a monitoring system and may contain a microprocessor for sensor fusion or for computing alarm conditions. The sensor unit can incorporate batteries and/or solar cells for as a power source, and communicates with the communication unit using a wireless communications link.
WO-A1-2009121181 discloses a method and system for determining a level of a substance in a container, the method comprises emitting one pulse from a light source in a field of illumination toward a surface of said substance in said container. A backscatter signal of said pulse is detected by an optical detector. A lidar trace is created from said backscatter signal, said lidar trace including at least one reflection peak; A surface reflection is identified among said at least one reflection peak in said lidar trace, said surface reflection being a reflection of said pulse from said surface. The surface reflection is signal-fitted to provide a fitted surface trace. A level of said substance is determined in said container using said fitted surface trace. Only the level of the substance is computed.
EP-A1-2708859 discloses a system for determining volume of material in a tank and method for measuring the quantity of material in a tank such as a commodity air cart of an air seeder in which a sensor or sensors are used to measure the distance to the surface of the material. The distance data is then used to determine a surface profile of the material from which the volume of material is calculated. The volume is converted to weight using known material density information.
In view of the above background, there is a need for a new method and system that provides higher accuracy than previous multi-level measuring methods, reaching accuracies similar to load cells, and avoiding the calibration complexities allowing the installation in any container type by non-skilled people. The invention makes uses of high-resolution and wide field-of-view 3D cameras to acquire a depth map of the area observed and it makes use of 3D processing algorithms to calculate an accurate representation of the surface of the content within the container. It also provides several alternatives to automatically obtain or adjust some critical parameters used by the 3D processing algorithms.